Pelagosites (calcareous crust formed in splash zone) and coniatolites (supratidal tufa) composed of inorganic precipitates of aragonite and some high-Mg calcite have been described from Holocene deposits along the western Persian Gulf and elsewhere. Such diagenetic deposits are believed to be restricted to intertidal and supratidal environments, and are only rarely encountered in ancient carbonate rocks. Laminated encrustations, coated grains, and pseudostromatolites of presumed former aragonite mineralogy, all associated to some extent with encrusting marine fossils, are the dominant if not exclusive components of shallow-marine limestones in the Tansill Formation (back-reef facies of the Capitan) in New Mexico.

These deposits occur in shallow back-reef environments of possible hypersaline character. In landward directions, they are replaced by peritidal dolomites. The vertical and lateral occurrence of particular coniatolite-pelagosite structures is related to the hydraulics of the depositional environment in a manner somewhat similar to that which controls algal laminite and stromatolite distributions in modern and ancient deposits. Internally, the laminae of crystalline calcite in these deposits are interlayered with the alga Archaeolithoporella and encrusted by Tubiphytes. Petrographically, this crystalline calcite consists of square-tipped crystal ghosts and divergent fan-druses in neomorphic sparry mosaics identical to altered aragonites described from other ancient carbonates.

These deposits are similar to the coniatolites and pelagosites described from the Holocene, but represent the first reported occurrence of extensive diagenetic sedimentation of this type in ancient shallow-marine carbonate rocks.

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